Abstract:

The study was carried out in a natural fen mire in the Biebrza River Valley (NE Poland). Sedge Warblers avoided settling in an open sedge meadow: their territories were situated in places overgrown with reed and bushes, often close to canals. Population density ranged from 10 to 24 pairs/10 ha. The majority of nests were built on tufts of sedges or small trees. Egglaying period lasted from 8 May to 22 June. Clutches contained 4-6 eggs (mean = 5.2) and clutch size signiiicantly decreased over the season. Nest losses were caused only by predation and accounted for 14.8% (calculated with the Mayfield method). The average production of fledglings per nest was 3.9 and per successful nest 4.6. The tledgling probability per egg was 77.6%.